Scholarly article on topic 'Measurement of natural radioactivity in granites and its quartz-bearing gold at El-Fawakhir area (Central Eastern Desert), Egypt'

Measurement of natural radioactivity in granites and its quartz-bearing gold at El-Fawakhir area (Central Eastern Desert), Egypt Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — M.A.M. Uosif, Shams A.M. Issa, L.M. Abd El-Salam

Abstract The distribution of natural radionuclides (226Ra, 232Th and 40K) in Granites and its quartz-bearing gold at El-Fawakhir area (Central Eastern Desert, Egypt) were measured by using γ-ray spectroscopy [NaI (Tl) 3″ × 3″]. X-Ray Fluorescence technique was used for chemical analyses of the studied samples. The specific activity of 226Ra, 232Th and 40K values are in range (3 ± 0.5 to 43 ± 2 Bqkg−1), (5 ± 0.7 to 41 ± 2 Bqkg−1) and (128 ± 6 to 682 ± 35 Bqkg−1) respectively. The absorbed dose rates ranged from 13.8 to 58.4 nGy h−1, where the total effective dose rates were determined to be between 16.7 and 70.9 μSvy−1. The maximum external hazard index (Hex) is 0.3 nGyh−1. The calculated values of the excess lifetime cancer risks (ELCR) and annual effective dose rate values are in between (8.48 × 10−5 and 2.63 × 10−4) and (24.2 and 72.9 μSvy−1) respectively. Geochemically, the studied granites consist of major oxides, they are characterized by SiO2, K2O, Na2O, Al2O3, and depleted in CaO, MgO, TiO2, and P2O5. The average absorbed dose rate (Do) in air is 37.8 nGyh−1 for the whole studied samples, this value is about 3.78% of the 1.0 mSvy−1 recommended by (ICRP-60,1991) to the public, so there is no radiological risk for the workers in that area.

Academic research paper on topic "Measurement of natural radioactivity in granites and its quartz-bearing gold at El-Fawakhir area (Central Eastern Desert), Egypt"

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Measurement of natural radioactivity in granites and its quartz-bearing gold at El-Fawakhir area (Central Eastern Desert), Egypt

M.A.M. Uosifa>*, Shams A.M. Issa a'b, L.M. Abd El-Salam c

a Physics Department, Faculty of Sciences, Al-Azhar University (Assiut branch), Egypt b Physics Department, Faculty of Sciences, Tabuk, Saudi Arabia

c Geology Department, Faculty of Sciences, Al-Azhar University (Assiut branch), Egypt

ARTICLE INFO

ABSTRACT

Article history: Received 31 August 2014 Received in revised form 26 January 2015 Accepted 18 February 2015 Available online xxx

Keywords:

Natural radionuclides Granite rocks Radiological Hazards Excess lifetime cancer risks

The distribution of natural radionuclides (226Ra, 232Th and 40K) in Granites and its quartz-bearing gold at El-Fawakhir area (Central Eastern Desert, Egypt) were measured by using g-ray spectroscopy [NaI (Tl) 3" x 3"]. X-Ray Fluorescence technique was used for chemical analyses of the studied samples. The specific activity of 226Ra, 232Th and 40K values are in range (3 ± 0.5 to 43 ± 2 Bqkg"1), (5 ± 0.7 to 41 ± 2 Bqkg"1) and (128 ± 6 to 682 ± 35 Bqkg"1) respectively. The absorbed dose rates ranged from 13.8 to 58.4 nGy h_1, where the total effective dose rates were determined to be between 16.7 and 70.9 imSvy-1. The maximum external hazard index (Hex) is 0.3 nGyh-1. The calculated values of the excess lifetime cancer risks (ELCR) and annual effective dose rate values are in between (8.48 x 10~5 and 2.63 x 10~4) and (24.2 and 72.9 mSvy-1) respectively. Geochemically, the studied granites consist of major oxides, they are characterized by SiO2, K2O, Na2O, Al2O3, and depleted in CaO, MgO, TiO2, and P2O5. The average absorbed dose rate (Do) in air is 37.8 nGyh-1 for the whole studied samples, this value is about 3.78% of the 1.0 mSvy-1 recommended by (ICRP-60,1991) to the public, so there is no radiological risk for the workers in that area. Copyright © 2015, The Egyptian Society of Radiation Sciences and Applications. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

When the earth was formed four billion years ago, it contained many radioactive isotopes. Since then, all the shorter lived isotopes have decayed. Only those isotopes with very long half-lives (100 million years or more) remain, along with the isotopes formed from the decay of the long lived isotopes. Background radiation is the ubiquitous ionizing radiation

that people on the planet Earth are exposed to, including natural and artificial sources. Both natural and artificial background radiation varies depending on location and altitude. Every day, we ingest/inhale nuclides in the air we breathe, in the food we eat and in the water we drink. Radioactivity is common in the rocks and soil that make up our planet, in the water and oceans, and even in our building materials and homes. It is just everywhere (Uosif & Abdel-Salam, 2011).

* Corresponding author. E-mail address: dr_mohamed_amin@lycos.com (M.A.M. Uosif).

Peer review under responsibility of The Egyptian Society of Radiation Sciences and Applications. http://dx.doi.org/10.1016/j.jrras.2015.02.005

1687-8507/Copyright © 2015, The Egyptian Society of Radiation Sciences and Applications. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Granite's durability and decorative appearance make it a popular building material in homes and buildings. Any type of rock could contain naturally occurring radioactive elements like radium, uranium and thorium. Some pieces of granite contain more of these elements than others, depending on the composition of the molten rock from which they formed. Geologists provide an explanation of this behavior in the course of partial melting and fractional crystallization of magma, which enables U and Th to be concentrated in the liquid phase and become incorporated into the more silica-rich products. For that reason, igneous rocks of granitic composition are strongly enriched in U and Th (on an average 5 ppm of U and 15 ppm of Th), compared to the Earth's crust (average 1.8 ppm for U and 7.2 ppm for Th) (Mason & Moore, 1982), the upper continental crust (average 2.7 ppm for U and 10.5 ppm for Th) (Rudnick & Gao, 2003) and rocks of basaltic or ultramafic composition (0.1 ppm of U and 0.2 ppm of Th) (Faure, 1986) and (Me'nager et al. 1993).

The Fawakhir granite is a stock intruded into the older Precambrian rocks. As no agriculture has ever succeeded in this hyperarid desert, the only resources are mineral, namely, gold, granite, and water. The granite was quarried to no great extent in the Roman period, but it also acts as an aquifer, carrying water in tiny cracks until it is stopped by the dense ultramafic rocks to the west. Most importantly, however, the quartz veins injected into the granite are auriferous, particularly towards the edge of the stock. The present work deals with the radioactivity and radiological hazard of El Fawakhir mining area.

2. Geological setting

El-Fawakhir granite platoon hosts and El-Fawakhir gold mines, which are two of several gold mines in the Eastern Desert of Egypt that have been extensively worked since Pharaonic and Roman times (Amer, Kusky, & Ghulam, 2008). The rock varieties encountered from the oldest to the youngest are meta-sediments, gabbroic sheet, younger granites and Quaternary Wadi deposits. The Carbonic sheet is grayish brown in color on the weathered surface and intruded by younger granites to the central part of the studied area. The volcanic rocks (40%) are located around El-Fawakhir granites and running east to west, Fig. 1.

The Gold pockets in the El-Fawakhir quarry in the Eastern side of the studied area. There are two points of view regarding the genesis of the known gold mineralization in the Eastern Desert of Egypt. Some authors discussed the gold mineralization genesis based on the geosynclinals theory (El-Shazly, 1956; Sabet, Tosgeov, Bordonosov, Baburin, & Zalta, 1976). Recently, some authors linked the gold mineralization to the plate tectonic theory (Hassan, 2006; Hassan, Azzaz, Soliman, El-Badway, 1991) under ophiolite, island arc and cordilleran-extensional, group. Each possesses characteristic ore mineral assemblage and geochemical association. Several Gold deposit and plugs are recorded in the studied area. The Gold pockets in the El-Fawakhir quarry in the Eastern side of the studied area. There are two points of view regarding the genesis of the known gold mineralization in the Eastern Desert of Egypt. Some authors discussed the gold

mineralization genesis based on the geosynclinals theory (El-Shazly, 1956). Some authors linked the gold mineralization to the plate tectonic theory (Hassan, 2006; Sabet et al., 1976) under ophiolite, island arc and cordilleran-extensional, group. Each possesses characteristic ore mineral assemblage and geochemical association.

Veins: The study area is traversed by gold-bearing quartz veinlets. The main occurrences are those at El Fawakhir main quartz veins are exploited for gold. These quartz veinlets represent the second stage of the heat engine process affecting the area and lead to the accumulation of gold along two stages of formation. The first stage accompanied the granitic intrusion where the mafic ultramafic rocks were heated and gold was mobilized to the heat source. The second stage is the intrusion of these quartz veinlets. The ore bodies of the vein type represent fissure fillings with some wall rock alterations in their vicinity.

2.1. Geochemistry

The chemical analysis for the granite samples under investigation has been done in order to identify their geochemical behavior. The chemical analysis for the major oxides (wt %) were done, using wet chemical analysis technique. The trace elements (ppm) were measured by XRF technique. All these chemical analyses were done in the Nuclear Materials Authority of Egypt. Table 1 shows the chemical analysis for major oxides and trace elements as well as some geochemical ratios for the studied monzogranites and syenogranites. The studied granites are, generally, characterized by their relatively high silica and Ba contents. Generally, the monzogranites are characterized by their relatively higher contents of FeO, Fe2O3, MgO, MnO, CaO, Sr and Ba as well as lower SiO2, K2O, Rb, Zr, Y, Nb and F- contents than those of the syenogranite.

The potassic feldspars are responsible for the increase in Ba and Rb, while zircon is responsible for high concentration of Hf content. The gold mineralization hosted in these granodiorites is responsible for the higher contents of ore elements, Cr, Co, Ni, Cu, Zn, As, Sn, Sb, confirming that these are evidence for elements form the geochemical association of the gold mineralization.

3. Experimental procedure and methods

3.1. Sampling preparation

Twenty samples (12 quartz-bearing gold and 8 granite samples) were collected from investigated area. The samples were crushed, homogenized and sieved through a 200-mm mesh, which is the optimum size enriched in heavy minerals. Each sample was dried in an oven a 110 °C to ensure that moisture was completely removed. Weighed samples were placed in a polyethylene beaker, of 350 cm3 volume each. The beakers were completely sealed for 4 weeks to reach secular equilibrium where the rate of decay of the progeny becomes equal to that of the parent (radium and thorium) within the volume and the progeny will also remain in the sample (ASTM, 1983; 1986).

Fig. 1 - Geological and location map of El-Fawakhir area.

Table 1 - Major elements (oxides wt.%) and trace elements (ppm) of the studied granotoid.

S. No G1 G2 G3 G4 G5 G6 G7 G8

SiO2 73.80 65.00 66.00 74.80 65.35 63.34 74.00 74.00

TiO2 0.20 0.92 0.86 0.14 0.60 0.70 0.05 0.03

Al2O3 14.00 16.76 16.61 13.33 16.49 16.23 14.30 13.75

Fe2O3 1.40 2.55 2.00 1.13 1.85 3.51 1.11 1.31

FeO 0.45 2.31 2.54 0.23 3.00 1.33 0.33 0.53

MnO 0.07 0.15 0.22 0.02 0.17 0.08 0.11 0.02

MgO 0.05 1.46 1.41 0.22 1.56 3.81 0.10 0.07

CaO 0.40 3.33 2.53 0.51 3.98 4.40 0.40 0.51

Na2O 4.00 4.00 3.77 4.10 3.00 4.22 3.95 4.43

K2O 4.77 4.54 4.00 3.97 4.61 4.43 4.12 4.36

P2O5 0.01 0.26 0.25 0.11 0.21 0.22 0.10 0.01

L.O.I 0.70 0.45 0.74 0.70 0.93 0.50 0.60 0.80

Total % 99.85 99.73 99.93 99.26 99.75 99.77 99.17 99.82

Trace elements (ppm)

Au 2.01 1.6 1.57 1.9 1.41 2.20 1.67 1.73

Cr 21 21 22 14 15 15 18 16

Ni 11 8 7 5 6 7 5 7

Cu 10 10 11 11 11 10 13 10

Zn 57 55 57 70 43 35 54 58

Zr 165 100 158 193 124 94 147 115

Rb 70 70 69 53 83 60 56 71

Y 23 15 14 16 16 10 10 13

Ba 456 555 435 519 480 765 400 546

Pb 11 14 14 6 13 12 14 11

Sr 525 663 330 428 306 461 571 397

Ga 17 15 14 17 14 12 17 18

V 15 10 11 27 7 6 11 9

U ppm 6.4 4.9 5.5 4.9 4.8 5.2 5.5 6.6

Th ppm 88 24.3 32.7 38 34.12 27 45 42

Instrumentation and calibration

Activity measurements were performed by gamma ray spectrometer, employing a scintillation detector (3" x 3"). It is hermetically sealed assembly, which includes a Nal (Tl) crystal, coupled to PC-MCA, CANBERRA Company (USA). To reduce gamma ray background, a cylindrical lead shield (100 mm thick) with a fixed bottom and movable cover shielded the detector. The lead shield contained an inner concentric cylinder of copper (0.3 mm thick) in order to absorb X-rays generated in the lead. In order to determine the background distribution in the environment around the detector, an empty sealed beaker was counted in the same manner and in the same geometry as the samples. The measurement time of activity or background was 43,200s. The background spectra were used to correct the net peak area of gamma rays of measured isotopes. The online analysis of each measured gamma-ray spectrum has been carried out by a dedicated software program (Genie, 2000) from Canberra Company (USA).

3.2.1. Calculation of activity

Calculations of count rates for each detected photo peak and radiological concentrations (activity per mass unit or specific activity) of detected radionuclides depend on the establishment of secular equilibrium in the samples. The 232Th concentration was determined from the average concentrations of 212Pb (238.6 keV) and 228Ac (911.1 keV) in the samples, and that of 226Ra was determined from the average concentrations of the 214Pb (351.9 keV) and 214Bi (609.3 and 1764.5 keV) decay products. The activity concentration in Bqkg-1 (A) in the environmental samples was obtained as follows (Shams, Mohamed, & Elsaman, 2012):

e x h ■

where Np = the (cps) sample - (cps) B.G, e is the abundance of the g-line in a radionuclide, h is the measured efficiency for each gamma-line observed for the same number of channels either for the sample or the calibration source, and m the mass of the sample in kilograms. The minimum detectable activity concentrations were 25.2 Bqkg-1 for40K, 6.5 Bqkg-1 for 226Ra and 5.7 Bqkg-1 for 232Th. All procedures were described in previous publication (Uosif, 2007).

3.2.2. Radium equivalent activity

Radium equivalent concentration (Raeq) is a common index used to compare the specific activities of materials containing 226Ra, 232Th, and 40K. It can be expressed It can be expressed (UNSCEAR 1982) as:

where, ARa, ATh and AK are specific activities (Bqkg-1) of 226Ra, 232Th and 40K respectively, Radium equivalent concentration (Raeq) was calculated based on the estimation that 370 Bqkg-1 of 226Ra, 259 Bqkg^1 of 232Th and 4810 Bqkg^1 of 40K produce the same g-ray dose rate (Yu, Guan, Stoks, & Young, 1992).

3.2.3. Absorbed dose rates (D0)

The absorbed dose rates due to gamma radiations in air at 1 m above the ground surface for the uniform distribution of the

naturally occurring radionuclides (226Ra, 232Th and 40K) were calculated based on guidelines provided by (ICRP-60):

D. = 0.427CRa + 0.662CTh + 0.043CK

where CRa, CTh and CK are the concentration in (BqKg 1 of radium, thorium and potassium respectively (Shams et al., 2013).

3.2.4. External hazard index (Hex)

The external hazard index (Hex) due to the emitted gamma rays for each sample was calculated according to the following formula (UNSCEAR, 1988):

Hex —

CRa CTh CK

370 + 259 + 4810

where CRa, CTh, and CK are the activity concentration of 226Ra, 232Th, and 40K, respectively.

3.2.5. Annual effective dose rate (AEDR)

The gamma absorbed doses in nGy h_1were converted to annual effective dose in mSvy-1 as proposed by (UNSCEAR, 2000). The annual effective dose rate (AEDR) was calculated by using the following equation.

AEDR = ADRA x DCF x OF x T (5)

where ADRA, DCF, OF are absorbed dose rate in air (nGy h_1), dose conversion factor.

(0.7 Sv Gy_1), outdoor occupancy factor (0.2) and the time, respectively (8760 hy_1).

3.2.6. Excess lifetime cancer risk (ELCR)

Excess lifetime cancer risk (ELCR) was calculated also and listed in coulomb (5) in Table 3 by using the fowling equation (Taskin et al., 2009):

Table 2 - Activity concentrations of 226Ra, 232 Th and 40K

in different samples.

Sample name Activity in Bqk g-1

226Ra 232Th 40K

Q1 21 ± 1 34 ± 2 304 ± 15

Q2 14 ± 1 30 ± 1 155 ± 8

Q3 10 ± 1 38 ± 2 159 ± 8

Q4 32 ± 2 41 ± 2 140 ± 7

Q5 14 ± 2 17 ± 3 229 ± 14

Q6 37 ± 3 22 ± 3 282 ± 16

Q7 29 ± 1 12 ± 1 663 ± 33

Q8 25 ± 1 35 ± 2 595 ± 30

Q9 23 ± 2 20 ± 4 415 ± 22

Q10 33 ± 2 14 ± 2 201 ± 12

Q11 29 ± 2 25 ± 3 682 ± 35

Q12 12 ± 1 15 ± 1 221 ± 12

G1 15 ± 1 15 ± 1 128 ± 6

G2 15 ± 1 16 ± 1 319 ± 16

G3 36 ± 3 28± 391 ± 20

G4 43 ± 2 27 ± 1 388 ± 19

G5 35 ± 3 29 ± 3 374 ± 19

G6 15 ± 2 9 ± 1 174 ± 10

G7 35 ± 3 13 ± 2 212 ± 12

G8 37 ± 3 22 ± 3 282 ± 16

Table 3 - Radium equivalent (Raeq), the dose rate (Do), external hazard indices (Hex), annual effective dose rate (AEDR), and excess lifetime cancer risk (ELCR).

Sample name Raeq (Bqkg-1) Do (nGyh-1) Hex (nGyh"1) AEDR mSvy"1 ELCR

Q1 91.0 44.6 0.3 54.1 1.89 x 10"4

Q2 67.1 32.2 0.2 39.1 1.37 x 10"4

Q3 75.5 36.3 0.2 44.1 1.54 x 10"4

Q4 45.5 21.9 0.1 26.6 9.30 x 10"5

Q5 54.2 27.0 0.2 32.8 1.15 x 10"4

Q6 59.5 30.4 0.2 36.9 1.29 x 10-4

Q7 92.6 48.9 0.3 59.3 2.08 x 10"4

Q8 116.7 59.4 0.3 72.9 2.63 x 10"4

Q9 81.3 41.2 0.2 50.0 1.75 x 10"4

Q10 40.3 20.0 0.1 24.2 8.48 x 10"5

Q11 112.8 58.4 0.3 70.9 2.48 x 10"4

Q12 48.9 24.6 0.1 30.1 8.85 x 10"5

G1 100.3 46.8 0.3 56.8 1.99 x 10-4

G2 87.6 42.2 0.2 51.3 1.80 x 10"4

G3 104.3 51.1 0.3 62.1 2.17 x 10"4

G4 108.4 52.7 0.3 64.1 2.24 x 10"4

G5 102.5 50.2 0.3 60.9 2.13 x 10-4

G6 67.4 32.2 0.2 39.1 1.37 x 10"4

G7 69.0 32.9 0.2 39.9 1.40 x 10"4

G8 87.6 42.2 0.2 51.3 1.80 x 10"4

ELCR = AEDE x DL x RF

where AEDE is the annual effective dose equivalent, DL is duration of life (70 year) and RF is risk factor (Sv-1) fatal cancer risk per Sievert. For stochastic effects, ICRP 60 uses values of 0.05 for the public.

4. Results and discussion

This study is a continuation of our ongoing project in physics department (Faculty of Science, Al-Azher University, Assuit Branch, Egypt) related to the measurement of specific activity of 226Ra, 232Th and 40K in environmental samples from Upper Egypt using a gamma-ray spectrometric technique and estimation of the gamma dose rate from these radionuclides. The obtained results of the measured specific activities in different samples which listed in Table 2 show that the values the highest 226Ra, 232Th and 40K concentrations are 43 ± 2, 41 ± 2 and 682 ± 35 (BqKg-1) respectively and the minimum concentrations are 10 ± 1, 9 ± 1 and 128 ± 7 (BqKg-1) respectively. The variations of natural radioactivity levels at different samples are due to the variation of concentrations of these elements in the geological formations. The obtained results for 226Ra are lower than the average international radioactivity levels, which is 35 Bqkg-1 (UNSCEAR, 1993) except in sample G5 reaching 43 Bqkg-1. The 232Th results are lower in all locations than the average international radioactivity levels of 50 Bqkg-1. The content of Th generally increases with SiO2 content and closely follows U during differentiation or partial melting. The increase in U with both SiO2 and alkali content is usually more marked than the increase in Th. The 40K results are also lower in all locations than the average international radioactivity levels of 500 Bqkg-1 except in three quartz

samples, which named Q7,Q8 and Q11 reaching 663, 595, and 682 respectively Bqkg"1.

Generally the presence of such high radioactivity in the granites may be attributed to the presence of relatively increased amount of accessory minerals such as zircon, iron oxides, fluorite and other radioactive related minerals. The radioactive related minerals play an important role in controlling the distribution of uranium and thorium. Zircon usually contains uranium and thorium concentration ranging from 0.01% to 0.19%, 1%-2%, respectively.

The obtained results of Raeq in Table 3 showed that, the lowest value Raeq was 40.3 Bqkg"1 where the highest one was 112.8 Bqkg"1. The maximum value (UNSCEAR 1993) of Raeq in granite samples must be less than 370 Bqkg"1 to keep the external dose below 1.5 mSvy_1.These values are less than the maximum admissible (UNSCEAR, 2000) value of 370 Bqkg"1.

The absorbed dose rate values which extracted from the concentration values of 226Ra, 232Th and 40K in (BqKg"1) were listed in Table 3. It shows that the lowest dose rate is 20 nGyh"1, while the highest dose rate was 58.4 nGyh"1 for granite samples. Studies indicate an average outdoor terrestrial gamma dose rate is 60 nGyh"1, it is lower than the global average values ranging from 10 to 200 nGyh"1 (UNSCEAR, 2000). In the other hand the calculated external hazard index (Hex) was found to be less than unity as shown in Table 3.

The calculated annual effective dose rate (AEDR) values were varied from 24.2 to 72.9 mSvy"1, these values are in the same global average outdoor terrestrial radiation value of 0.07 mSv y 1 reported by UNSCEAR, 2000. The highest excess lifetime cancer risk value for the granite samples as shown in Table 3 was 2.5 x 10~4, while the lowest ELCR value was 8.48 x 10~5 with average value (1.61 x 10~4), this value is lower than average world value (2.9 x 10~4) (ICR-90, 1990). Yet we were not able to evaluate the health hazards of the assessed values on the population. Since reliable, standardized

mortality and morbidity statistics were not accessible, this study was limited to background radiation levels.

5. Conclusion

The values of measured specific activities of natural radio-nuclides 226Ra, 232Th and 40K in investigated samples are within the acceptable limits. The average values of all the calculated radiological indices extracted from these activities, in all investigated samples are within the levels recommended by UNSCEAR, 2000 report. The data obtained here are reference values to be used as a data baseline for drawing a radiological map of El-Fawakhir area.

Acknowledgment

This work was carried out using the nuclear analytical facilities at the Physics department, Faculty of Sciences, Al-Azhar University, Assiut, Egypt.

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